gas: ld: libsframe: Support for SFrame FDEs without any FREs

gas: ld: libsframe: Support for SFrame FDEs without any FREs

Allow SFrame sections without any FREs, that can occur if they solely
contain FDEs without any FREs.  For FDEs without and FREs set the
offset to the first FRE to zero.

libsframe/
	* sframe.c (sframe_encoder_write_sframe): Allow SFrame sections
	without any FREs.  For FDEs without any FREs set the offset to
	the first FRE to zero.

gas/
	* gen-sframe.c (output_sframe_funcdesc): For FDEs without any
	FREs set the offset to the first FRE to zero.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>